1. Field of the Invention
This invention relates to improvements in and relating to a leaf spring element which is formed by rolling a strip of spring steel and a laminated leaf spring including a plurality of such leaf spring elements. The conventional leaf springs of this sort have a flat shape in cross-section along a plane perpendicular to the lengths thereof so that, upon exerting of a bending moment, concentrated stress occurs in corner edge portions of the loaded surface on the tension side, which can be the cause of fatigue failure in these corner edge portions. The present invention contemplates providing a leaf spring element which can be easily fabricated without incurring substantial changes in the conventional manufacturing methods for ordinary flat rolled steel strips and which is lightweight, high in fatigue strength and low in production cost. The present invention also provides a laminated leaf spring using a plurality of such leaf elements for its lamellar layers.
2. Description of Prior Art
In the production of leaf springs for the suspensions of vehicle axle shafts, it has been the general practice to use ordinary flat rolled steel strips which have a substantially rectangular shape in cross-section along a plane perpendicular to the longitudinal direction of the steel strip as shown in FIG. 1. In some cases a rolled flat steel strip of a channel-like shape with a groove on the bottom surface is employed, as shown in FIG. 2 in cross-section along a plane perpendicular to the length of the steel strip. A trapezoidal shape is shown in FIG. 3 in similar cross-section. A steel plate of the sectional shape shown in FIG. 2 or 3 is used for the leaf spring because it has a broader width on the tension stress side than on the compression stress side, as compared with with the steel strip of a plain rectangular cross-section shown in FIG. 1, so that the neutral axis of the sectional area is shifted toward the tension stress side under a bending load thereby reducing the tension stress (increasing compression stress) under a bending moment and increasing the amplitude of the bearable repeated bending moment per unit weight. For example, a laminated leaf spring including a number of leaf elements of the cross-sectional shape shown in FIG. 2 or 3, which is widely used for the suspension of an axle shaft for vehicles, is subjected to a mean bending moment of static load as well as an amplitude of bending moment of dynamic load, and improvements in the fatigue strength per unit weight have been attempted by utilizing the fact that the fatigue strength is higher on the compression stress side of the beam than on the tension stress side under a mean bending moment by static load.
However, in comparison with the steel strip of a simple rectangular cross-sectional shape shown in FIG. 1, greater difficulties are involved in rolling leaf elements of a cross-sectional shape shown in FIG. 2 or 3 within a prescribed tolerance of errors, in addition to higher rolling costs. Further, the leaf elements with the cross-sectional shapes shown in FIGS. 1 to 3, have a common problem in that a fatigue fracture is apt to occur at the corner edge portions of the transverse cross-sectional shape on the tension stress side where concentration of stress takes place, exhibiting a fatigue strength about 20% lower than that of a round bar spring (spring having round cross-sectional shape) which has no such critical edge portion, as exemplified in FIG. 4. More specifically, FIG. 4 is a diagram taking the endurance limit as the ordinate and hardness as the abscissa, in which the endurance limit under rotating bending of a round bar spring is shown by a solid black circle and the endurance limit under plane bending of a flat steel spring is shown by a blank circle.